The HVS in collaboration with SmithKline Beecham, Rixensart, Belgium, has developed several candidate live attenuated HAV vaccines. In addition, the HVS has developed a candidate recombinant hepatitis E vaccine that is highly promising and that is currently in clinical trials. The HVS is studying the technology of DNA vaccines with a model system based upon hepatitis B virus (HBV) vaccine, a vaccine with which the HVS has had extensive experience. We have tested the efficacy of an immunostimulant (CpG) as an adjuvant for DNA vaccines, as well as for protein vaccines. In addition, the utility of DNA vaccines for the control of hepatitis C virus (HCV) has been explored. A DNA vaccine based on the E2 envelope protein of HCV proved to be highly immunogenic in mice and rhesus monkeys and moderately immunogenic in chimpanzees, but the chimpanzees were not fully protected when they were challenged with live HCV. A similar approach has been utilized in the study of a DNA vaccine based on the E1 envelope protein of HCV. Various constructs of the E1 gene were prepared as DNA vaccines (expression vector plasmids) and as vectored vaccines (recombinant vaccinia). Mice were vaccinated with the DNA vaccine ("prime"), followed by vaccination with the recombinant vaccinia viruses ("boost"). The mice had excellent immune responses to the DNA vaccine as well as to the vaccinia boost. Based on these results, the studies will be extended to non-human primates, including chimpanzees, to determine if such immune responses are protective. Passive immunoprophylaxis has also been an important public health tool. For example, normal immunoglobulin has been important in the prevention of hepatitis A. However, monoclonal preparations could be more potent, tailored to specific neutralization epitopes and highly consistent in potency. We have prepared combinatorial libraries from the bone marrow of chimpanzees that had been experimentally infected in sequence with each of the five human hepatitis viruses. Chimpanzee globulins are virtually identical to human immunoglobulins, making them attractive choices for immunoprophylactic and immunotherapeutic agents. To date, we have isolated monoclonal immunoglobulins that react with HAV, HBV, HDV and HEV. In other studies, we have recovered human monoclonal antibodies that react with HCV. Many of the monoclonal antibodies described above are neutralizing and their production is being scaled up for tests of passive immunoprophylaxis in chimpanzees. Similar construction of combinatorial libraries from bone marrow is being carried out for chimpanzees that have been experimentally infected with dengue viruses 1 through 4 and the Norwalk virus, one of the most common causes of adult viral gastroenteritis. In some cases (dengue fever) recombinant monoclonal antibodies may prove to be important prophylactic and therapeutic agents. In other cases (Norwalk virus), the monoclonal antibodies will be useful as diagnostic reagents. We plan to extend these studies to other viruses of interest that can be experimentally administered to chimpanzees.